Microwave circuitry is most easily manufacturable in microstrip and in stripline implementations. Microstrip circuitry has a single dielectric substrate layer with a ground plane on one side and microstrip conductors on the other side. Stripline circuitry has two dielectric substrate layers with the conductor array pattern sandwiched therebetween, and ground planes on the outer surfaces.
In either microstrip or stripline implementations, -3 dB couplers are not cost efficient manufacturable. This is because the coupling gap is too small, approaching zero. In either the Lange type interdigitated form or in a three level form, -3 dB couplers have thus been implemented. Even Lange type and three level type -3 dB couplers, however, involve considerable manufacturing cost.
A Lange type interdigitated -3 dB coupler, for example as shown in "Interdigitated Strip-Line Quadrature Hybrid", Julius Lange, 1969 International Microwave Symposium, Dallas, Texas, May 5-7, IEEE Cat. No. 69 C 6, pp. 10-13, employs a plurality of parallel interdigitated coupler lines spaced by narrow gaps. For high dielectric constant substrate material, for example alumina, i.e., aluminum oxide, the gap width is about 1 to 2 mils. These narrow gap widths and the plurality of conductor lines substantially increase manufacturing cost. On low dielectric constant substrate material, for example Teflon glass having a dielectric constant of about 2.2, the gap width would have to be on the order of 0.5 mil. This extremely narrow gap is even more difficult to fabricate, and from a pragmatic standpoint is probably not manufacturable, within reasonable limits of cost efficiency.
The other type of -3 dB coupler, the three level type, employs three dielectric substrate layers. The middle layer is sandwiched between conductor coupling lines, which are in turn sandwiched between the outer substrate layers, which are in turn sandwiched between outer ground planes. This structure is bulky, costly and difficult to incorporate with other microwave circuitry.